SANTA CRUZ, Calif., Feb. 22 (UPI) -- U.S. scientists said they have observed in detail a source of high-energy gamma rays that originate on Earth, not from outer space.
Scientists using NASA's High Energy Solar Spectroscopic Imager satellite said they have detected flashes of gamma-ray energy in the planet's upper atmosphere. The satellite was designed to study X-rays and gamma rays from solar flares, but it also can detect gamma rays from a variety of sources.
The scientists, at the University of California, Santa Cruz, and elsewhere said the energy bursts are called terrestrial gamma-ray flashes, or TGFs. They are very short blasts of gamma rays, lasting about one-thousandth of a second and emitted into space by electrons traveling at 99.99 percent of the speed of light.
Originally discovered in 1994, the origin of TGFs has remained a mystery. The best guess at present is they are generated by the build-up of electrical charges at the tops of thunder clouds due to lightning discharges.
TGFs have been correlated with lightning strikes, the scientists said.
